1. Field of the invention
The present invention relates to a solid electrolytic capacitor using a valve metal or its alloy as an anode and a production method thereof.
2. Description of the Related Art
Conventionally, there is known a solid electrolytic capacitor in which an anode formed of a valve metal or its alloy is anode-oxidized in an aqueous solution of phosphoric acid or the like, to form a metal oxide layer which is a dielectric member on the surface. On the dielectric layer formed by anode oxidation, an electrolyte layer formed of manganese dioxide or the like is formed, and this leads a problem of an increase in equivalent series resistance (ESR) because of a low conductivity of manganese dioxide compared to a metal or the like.
It has been proposed to reduce ESR by using a conductive polymer instead of manganese dioxide as an electrolyte layer. However, in a solid electrolytic capacitor in which a conductive polymer is used as an electrolyte layer, there is a problem that a leak current is large compared to a solid electrolytic capacitor using manganese dioxide as an electrolyte layer. In particular, in the case of a solid electrolytic capacitor in which niobium is used as an anode, an oxide layer which is a dielectric layer is susceptible to heat and weak to pressure, so that a problem arises in which a leak current increases in a step of forming an outer package for sealing a device.
In order to reduce such a leak current, Japanese Patent Application Laid-open Publication No. 9-246114 proposes to form an outer package, perform heat treatment for curing, make a capacitor device absorb moisture, perform aging treatment by applying voltage under high temperature, and dry the absorbed moisture after the aging treatment.
Japanese Patent Application Laid-open Publication No. 2001-267186 proposes to reduce a leak current by performing drying in at least two stages as a drying condition after aging treatment, thereby reducing deterioration in a dielectric oxide layer.
However, these methods complicate a manufacturing process, and have a problem that reduction in a leak current is insufficient.